FIG. 1 illustrates a Long-Term Evolution (LTE) cellular network 100 distributed over land areas 110 referred to as cells, each of which is served by a base station 120. The cells 110 are geographically joined together to enable LTE terminals 130 (e.g., mobile phones, laptops, tablets, etc) to wirelessly communicate over a wide area with a core network (not shown) via the base stations 120.
Before an LTE terminal can communicate over an LTE cellular network, such as the LTE cellular network 100 in FIG. 1, an LTE terminal typically needs to perform a cell search to acquire frequency and symbol synchronization to a cell and detect the physical-layer identity of the cell. For example, the LTE terminal can perform a cell search to acquire synchronization to the cell and detect the physical-layer identity of the cell in which it is located or some other cell. In addition, the LTE terminal can continuously perform the cell search to acquire synchronization to other nearby cells and detect the physical-layer identity of the other nearby cells. This allows the LTE terminal to move from one cell to another while maintaining substantial connectivity to the LTE cellular network. For example, if the signal quality supported by a current cell becomes less than the signal quality supported by one of the other nearby cells due to the movement of the LTE terminal, communications with the current cell can be handed off to the nearby cell supporting the higher signal quality.
Two synchronization signals—the Primary Synchronization Signal (PSS) and the Secondary Synchronization Signal (SSS)—are broadcast from base stations in the LTE cellular network to assist in the cell search. The time-domain position of these two signals within an LTE frame is generally constant from frame-to-frame to support synchronization and depends on whether the LTE cellular network is operating in a frequency-division duplexing mode (FDD) or a time-division duplexing mode (TDD). As shown in FIG. 2, the general LTE frame configuration 200 is ten milliseconds in duration and includes two, five millisecond half-frames. Each half-frame is further divided into five sub-frames (0-4 and 5-9) that are each one millisecond in duration. The sub-frames typically carry 14 orthogonal frequency division multiplexing (OFDM) symbols. In an LTE cellular network operating in the FDD mode, the PSS is typically transmitted in the last OFDM symbol of sub-frames 0 and 5, and the SSS is typically transmitted in the second to last OFDM symbol right before the PSS in the same sub-frames. In an LTE cellular network operating in the TDD mode, the PSS is typically transmitted in the third OFDM symbol of sub-frames 1 and 6, and the SSS is typically transmitted in the last OFDM symbol of sub-frames 0 and 5.
During the cell search, the LTE terminal uses the PSS and SSS to acquire frequency and symbol synchronization to a cell and detect a physical-layer cell identity of the cell. Detecting the physical-layer cell identity involves obtaining an identity NID1=(0, . . . , 167) of a group from the SSS sequence broadcast from a base station, and obtaining an identity NID2=(0, 1, 2) within the group identified by NID1 from the PSS sequence broadcast by the base station. The group identity NID1 is detected from the SSS sequence after the identity NID2 within the group is detected from the PSS sequence. After detecting NID1 and NID2, the physical-layer cell identity can be determined using the relationship NID=(3*NID1)+NID2, where NID is the physical-layer cell identity. Because there are 168 unique group identities NID1 and three unique identities NID2 within each group, there are a total of 504 unique physical-layer identities in an LTE cellular network.
Typically, the LTE terminal receives strongly powered synchronization signals (i.e., PSS and SSS) from some base stations and comparatively weakly powered synchronization signals from other base stations. The strongly powered synchronization signals can overwhelm the weakly powered ones at the LTE terminal, preventing the LTE terminal from acquiring synchronization to and detecting the physical-layer identity of the cells from which the weakly powered synchronization signals originate. Acquiring synchronization to and detecting the physical-layer identity of these cells can be beneficial for several reasons, including for handing off communications from one cell to another due to, for example, movement of the LTE terminal.
The embodiments of the present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.